Although lead has been traditionally used in numerous industrial applications, current regulations have required the phase out of lead in most commercial products. For example, the European Union issued regulations in 2006 that mandated the elimination of lead from coatings and solders used in most electronic components. Other countries have issued similar mandates.
Electronics and like connections made using lead-based soldering materials are typically very reliable, and large capital investments have been made in associated manufacturing infrastructure. The worldwide phase out lead-based soldering materials has raised serious concerns regarding the reliability of alternative soldering materials and methods. Although many alternatives to traditional lead-based soldering materials have been developed, the Sn/Ag/Cu (SAC) system being among the most widely used, such replacements have typically exhibited drawbacks that make them unsuitable for extreme environments such as those found in automotive, military and space vehicles, for example. Specifically, the SAC system has a significantly higher eutectic melting point (e.g., m.p. of ˜217° C.) than does traditional Sn/Pb solder (m.p. of 183° C. for 63/37 Sn/Pb or 188° C. for 60/40 Sn/Pb), thus limiting its use to materials that are capable of withstanding its higher processing temperature. Furthermore, silver is a relatively expensive component in the SAC system, and there is insufficient silver production capacity to totally replace lead-based soldering materials with the SAC system. From an economic standpoint, the SAC system can undesirably lead to significantly higher production costs due to the material cost of silver and the more robust components needed to withstand its higher processing temperature. Even more importantly, SAC systems are prone to formation of tin whiskers, thereby increasing the risk of electrical shorting.
Several compositions containing metal nanoparticles have been proposed as replacements for traditional lead-based soldering materials. Nanoparticles can exhibit physical and chemical properties that sometimes differ significantly from those observed in the bulk material. For example, metal nanoparticles having a size of less than about 20 nm can exhibit a fusion temperature that is significantly below the melting point of the bulk metal. Copper nanoparticles, in particular, can have a fusion temperature that is comparable to that of traditional lead-based soldering materials. When copper nanoparticles are about 10 nm or less in size, the copper nanoparticles can have a fusion temperature of about 200° C. or less, thereby providing processing temperatures that are comparable with traditional lead-based soldering materials. Copper nanoparticles can also be considered as replacements for high temperature soldering materials such as AuSn, since they provide an initial low fusion temperature and a significantly higher reflow temperature thereafter.
Although copper nanoparticles are of significant interest due to their compatibility with existing soldering methods, the formation of monodisperse copper nanoparticles remains synthetically challenging, particularly at bulk scales required for commercial production. Further, it can be difficult to reversibly protect copper nanoparticles in order to prevent their agglomeration with one another. Protection can sometimes be accomplished with a thin oxide coating or a surfactant, including polymers such as polyvinylpyrrolidone, but oftentimes these agents cannot be effectively removed in order that the copper nanoparticles can function as desired in soldering applications. In addition, these agents can introduce contaminants or can themselves be considered contaminants that detrimentally impact the properties of copper nanoparticles. Properties that can be impacted can include, for example, electrical and thermal conductivity, mechanical strength, brittleness and fracture toughness.
In view of the foregoing, scaleable processes for the synthesis of monodisperse metal nanoparticles, particularly copper nanoparticles having a size of 10nm or less, would be of substantial benefit in the art. The present invention satisfies this need and provides related advantages as well.